1. Field of the Invention
This invention relates to a picture signal processing method and apparatus used with advantage for recording/reproducing moving picture signals and still picture signals on or from a recording medium, such as an optical disc or a magnetic tape, and for transmitting moving and still picture signals from a sender to a receiver over a transmission path such as in a teleconferencing system, a television telephone system or in a broadcasting equipment.
2. Description of the Related Art
As an encoding system for efficient encoding of still or moving pictures, by way of data compression, there has so far been known a block encoding system, such as block discrete cosine transform (DCT) encoding system.
In companding picture data by encoding, the ringing noise (mosquito noise) or block distortion tends to be produced. The higher the compression ratio, the more readily the noise is liable to be produced. Also, the more intricate the input picture data, the more readily the noise is liable to be produced. The result is that deterioration due to the noise is produced in a picture corresponding to the picture data obtained on compression/expansion.
For reducing picture deterioration of the picture data due to noise produced on compression/expansion, it may be contemplated to use a pre-filter, such as a low-pass filter (LPF), and to perform filtering on the picture data corresponding to the input picture prior to encoding.
FIG. 1 shows an example of an encoding device for moving picture signals using the above-mentioned pre-filter.
In the encoding device, shown in FIG. 1, digital moving picture signals, entering a terminal 201, are filtered by a pre-filtering circuit 202. The filtered picture signals for processing are entered to an encoding circuit 205.
The encoding circuit 205 encodes digital moving picture signals using a hybrid encoding method combined from the motion-compensated inter-frame prediction and transform coding, such as DCT. That is, the encoding circuit 205 performs inter-frame/intra-frame adaptive prediction on an input signal S211, while performing DCT on prediction error signals by an arithmetic unit 206. A quantizer 208 quantizes calculated DCT coefficients to output an encoded output signal S215 at a terminal 220. It is noted that the quantizer 208 sets the quantization step size which will give a constant bitrate of the output signal S215.
The output signal S215 is dequantized by a dequantizer 209 and inverse DCTed by an inverse DCT unit 210 to restore prediction error signals to which prediction picture signals are added at an arithmetic unit 211 by way of local decoding. This addition gives decoded picture signals S213 which are stored in a frame memory 212.
In general, the more intricate the input moving picture or the more vigorous its motion, the larger becomes an inter-frame difference signal S212. If, for suppressing the volume of the generated information to a pre-set bitrate, coarse quantization is used at this time at the quantizer 208, perceptually outstanding block distortion is produced to deteriorate the apparent picture quality.
Thus, in the encoding device shown in FIG. 1, the low-pass passband of the pre-filter with respect to the input picture is restricted, in dependence upon the magnitude of the inter-frame difference signal of the input picture, responsive to the magnitude of the inter-frame difference signal of an input picture, in consideration that the magnitude of such inter-frame difference signal of the input signal affects the quantity of generated bits for inter-frame predictive coding. This attenuates the energy of the prediction error signals to prohibit coarse quantization to improve the apparent picture quality.
The variable pre-filter controlling method in the present encoding device is now explained.
In the arithmetic unit 204, an inter-frame difference r between a picture signal S210 entering the input terminal 201 and a picture signal S214 entering a memory 212 is calculated. The picture signal S214 is a signal to which reference is had in the inter-frame prediction for the input picture signal S210. The inter-frame difference r is entered to a pre-filter controller 203 which then generates a pre-filtering coefficient k, as a parameter for controlling the low-pass passband of the pre-filter, in dependence upon the magnitude of the inter-frame difference r. The pre-filter 202 has two-dimensional low-pass filter characteristics, shown in FIG. 2, with the low-pass passband showing monotonous increase with respect to the pre-filtering coefficient k. FIG. 3 shows the relation between the inter-frame difference r and the pre-filtering coefficient k. By the above relation, the pre-filter controller 203 controls the low-pass characteristics of the pre-filter depending on the magnitude of the inter-frame difference signal of the input picture.
As a technique for reducing the mosquito noise and the block distortion following the decoding, there is known a technique employing a system in which the pixel-based noise quantity is predicted from the macroscopic characteristics of the block level and from the microscopic characteristics of the pixel level, based on the local statistic quantity and the encoding information of the picture, and the noise component is removed by adaptive filtering.
As the technique for reducing the block distortion or the mosquito noise, there is known a post-filtering technique of filtering as-decoded output picture signals in addition to the above-described technique.
Although the above-mentioned pre-filtering processing, prior to encoding, is able to suppress the mosquito noise, the block distortion cannot be effectively removed especially in a picture performing vigorous movements. Moreover, the high frequency components are diminished by pre-filtering, such that, if a pre-filtered picture is decoded, only the block distortion is apparent to the eye.
In the post-filtering processing, performed following the decoding, there is no change caused in the encoding efficiency following the encoding, because no processing occurs on the decoding side, such that it is not possible to cope successfully with roughed quantization over an entire input picture performing vigorous movements difficult to predict.
That is, in a conventional picture encoding/decoding apparatus, the pre-filtering prior to the encoding and the post-filtering following the decoding are performed independently of each other, such that the noise cannot be removed effectively.
It is therefore an object of the present invention to provide a picture signal processing method and apparatus whereby it is possible to suppress the block distortion or the mosquito noise generated on encoding or decoding input moving or still pictures.
In one aspect, the present invention provides a picture signal processing apparatus for encoding and decoding an input picture on a block basis, including filtering means for filtering the input picture responsive to characteristics of the input picture, encoding means for block encoding a picture filtered by the filtering means, decoding means for decoding the picture encoded by the encoding means and noise reducing means for reducing at least the block distortion of the picture decoded by the decoding means.
With this picture signal processing apparatus, the input is filtered by a filtering means before being passed through the encoding means, while the picture decided by the decoding means is reduced in block distortion by noise reducing means.
With the picture signal processing apparatus, the input is improved in encoding efficiency by filtering, while it is reduced in the block distortion or mosquito noise otherwise produced on decoding.
Thus, with the present picture signal processing apparatus, the high frequency components of the input picture are reduced and the block distortion etc produced on decoding is rendered apparent to improve the detection accuracy of the block distortion etc. Thus, the noise can be reduced effectively by the noise reducing means following the decoding.
In another aspect, the present invention provides a picture signal processing method for encoding and decoding an input picture on a block basis, including filtering the input picture responsive to characteristics of the input picture, block encoding a filtered input picture, decoding the picture encoded by the block encoding and reducing at least the block distortion of the decoded picture.
With this picture signal processing method, the input picture is filtered before being encoded, while the block distortion in the picture is reduced following the decoding.
Thus, with the present picture signal processing method, the high frequency components of the input picture are reduced and the block distortion etc produced on decoding is rendered apparent to improve the detection accuracy of the block distortion etc. Thus, the noise can be reduced effectively following the decoding.